An electronic apparatus such as a mobile phone, a smartphone, a tablet information terminal, or a digital camera includes a display screen such as a liquid crystal display. In such electronic apparatus, the extent of approach of a human body to the apparatus is detected and various controls are performed. In addition, the display screen of the electronic apparatus also functions as a touch panel to input information.
Therefore, there is a case where a function of the touch panel needs to be turned off so that the electronic apparatus does not erroneously operate even if any object approaches the touch panel when a user performs an operation of starting a call or storing the electronic apparatus in a pocket. In addition, in the digital camera, such control that automatically lowers brightness of the display screen is performed when the user moves a visual line from the display screen to a viewfinder.
For such a purpose, a small and inexpensive optical photodetection device (object detection sensor) is used. The photodetection device determines the extent of approach of an object to be detected, by detecting pulsed light which is emitted by the photodetection device and reflected from the detected object. In the field of the photodetection device, a photodetection device which is used for such a purpose is generally called a proximity sensor.
In addition, in an office electronic apparatus, such as a copying machine or a printer, a photodetection device, which operates on the same principle as in the proximity sensor, is used in order to detect rotations, origin points or endpoints of various movable mechanical units, or in order to sense existence/non-existence of paper in a specific spot. Specifically, the photodetection device detects or determines existence/non-existence of light emitted by the photodetection device and reflected from the detected object or light emitted by the photodetection device and transmitted through a transmitting part of the detected object. In the electronic apparatus, control for each unit is performed based on a result of the detection performed by the photodetection device. In the field of the photodetection device, the photodetection device which is used for such a purpose is generally called a photo-interrupter.
The present invention is targeted, in the photodetection devices described above, at particularly a photodetection device which detects determines existence/non-existence of the detected object by detecting pulsed light which is emitted by the photodetection device and is reflected from the detected object, and a photodetection device which detects or determines the existence/non-existence of the detected object or a thickness of the detected object using light transmitted between a light emitting element and a light receiving element.
It is obvious to those skilled in the art that, in a case where the pulsed light is used as a signal, a degree of freedom in design is acquired with high resistance to disturbance light noise under various environments as compared to a case where signal light is DC light. The disturbance light noise includes sunlight, lighting equipment such as incandescent light, inverter fluorescent light, or the like, white LED light PWM-modulated, and the like. In addition, it is obvious to those skilled in the art that, in a case where the light emitting element is driven with pulses, it is possible to reduce power consumption as compared to a case where the light emitting element is driven in a DC manner.
In addition, in a case where the photodetection device is mounted on an electronic apparatus such as a copying machine or a personal computer which is connected to an alternating current power source for home use, or an electronic apparatus which has a motor or the like that is a noise generation source, it is necessary to increase resistance to a malfunction of the photodetection device due to high frequency noise which is superimposed on a power source line.
A typical pulse-emission photodetection device will be described. The photodetection device includes a light emitting element, a light receiving element, and a circuit element. The circuit element includes a light emitting element drive circuit, an IV conversion circuit (current voltage conversion circuit), an amplification circuit, a comparator circuit, a signal processing circuit, a timing generation circuit, and the like. Typically, for cost reduction, the light receiving element and the circuit element except the light emitting element among the above-described circuits are integrated on one semiconductor substrate in many cases.
FIG. 8 illustrates an example of a configuration of a typical photodetection device 501.
As illustrated in FIG. 8, in the photodetection device 501, in many cases, a differential amplification circuit 51 is generally used as the amplification circuit in order to improve resistance to a malfunction due to, for example, high frequency noise which is superimposed on the power supply line. An output terminal of the light receiving amplifier circuit 54 is connected to a positive input terminal of the differential amplification circuit 51 through a first high-pass filter circuit 55. The light receiving amplifier circuit 54 includes a light receiving element 52, such as a photodiode, and an IV conversion circuit 53 that has an input terminal to which the light receiving element 52 is connected. An output terminal of a dummy amplifier circuit 59 is connected to a negative input terminal of the differential amplification circuit 51 through a second high-pass filter circuit 60. The dummy amplifier circuit 59 includes a dummy light receiving element 57 that has a light receiving surface which is shaded by a metal layer or the like, and an IV conversion circuit 58 (which has the same configuration as the IV conversion circuit 53) that has an input terminal to which the dummy light receiving element 57 is connected.
In the differential amplification circuit 51, a difference voltage (=light receiving amplifier output voltage−dummy amplifier output voltage) between voltages, which are respectively input t the positive input terminal and the negative input terminal, is amplified. With such a differential configuration, high frequency noise or the like, which is superimposed on respective signal lines of the light receiving amplifier circuit 54 and the dummy amplifier circuit 59 in phase, is removed from the power supply line or the like in phase. Therefore, a malfunction of a photodetection device 501 due to noise superimposed on the power supply line, or the like is suppressed. Furthermore, even in the differential configuration, reflected light or transmitted light is not incident on a light receiving surface of the dummy light receiving element 57, and thus a normal pulse signal not reduced. In a photodetection device disclosed in PTL 1, a similar differential configuration is used.
The photodetection device 501 which is configured as described above operates as follows. First, in a case where the light emitting element 62, such as an LED, is caused to emit pulsed light based on a light emitting element drive pulse (A) by the light emitting element drive circuit 61, the light receiving element 52 receives reflected light (B) that is light from the light emitting element 62 and reflected by a detected object 63. Alternatively, in a configuration in which light is emitted from the light emitting element 62 toward the light receiving element 52, light that is emitted from the light emitting element 62 and transmitted through the detected object 63 is received using the light receiving element 52.
A current pulse signal, which is output from the light receiving element 52, is converted into a voltage pulse signal (light receiving amplifier output (C)) by the IV conversion circuit 53 (current voltage conversion circuit). The voltage pulse signal and a dummy amplifier output (D) from the dummy amplifier circuit 59 are differentially amplified by the differential amplification circuit 51 (differential amplification circuit output (E)). In the comparator circuit 64, the differential amplification circuit output (E) is compared with a predetermined threshold voltage (F). In a case where an amplitude value of the differential amplification circuit output (E) exceeds the threshold voltage (F), the comparator circuit 64 outputs a digital pulse signal (comparator output (G)) to the signal processing circuit 65. As illustrated in FIG. 9, the signal processing circuit 65 includes an AND circuit 65a and an SR latch circuit 65b as a configuration for synchronous detection, and includes an AND circuit 65c and an SR latch circuit 65d as a configuration for non-synchronous detection.
In the signal processing circuit 65, whether or not the output signal of the comparator circuit 64 coincides with timing of the light emitting element drive pulse (A) (synchronous detection gate signal (H)) is detected by the AND circuit 65a and the SR latch circuit 65b (synchronous light detection). The existence/non-existence of the detected object 63 or the like is determined according to a result of the detection. In addition, in a case where high disturbance light noise resistance and high power supply line noise resistance are desired, whether or not comparator output (G) is output at timing other than the light emitting element drive pulse (A) by the AND circuit 65c and the SR latch circuit 65d in the signal processing circuit 65 is detected (non-synchronous light detection). A non-synchronous detection gate signal (I) is output at timing other than the light emitting element drive pulse (A). There is a case where the existence/non-existence of the detected object the like is determined through combination of a result of the synchronous light detection and a result of the non-synchronous light detection.
A result of determination performed by the signal processing circuit 65 is output, as detection or non-detection, to an external output terminal through the output circuit 66.
The timing generation circuit 67 is composed of a logical circuit, and generates various timing signals based on an output signal (clock signal (P)) of the oscillation circuit 68. Specifically, the timing generation circuit 67 generates a measurement cycle (cycle of light emission, detection, output, or the like), the light emitting element drive pulse (A), the gate signal (the synchronous detection gate signal (H) or the non-synchronous detection gate signal (I)), which is synchronous or non-synchronous with a light emission pulse for synchronous or non-synchronous detection, a reset signal (L) which is used to initialize each of the circuit elements, and the like.
FIG. 10 illustrates a detailed example of operational waveforms of the reflected-light-detection photodetection device 501 according to the related art. FIG. 10 illustrates waveforms of three cycles corresponding to a measurement period, illustrates respective waveforms up to two cycles on the left side in a case where the detected object 63 exists, and illustrates each waveform in one cycle on the right side in a case where the detected object 63 does not exist.
The clock signal (P), which is output from the oscillation circuit 68, includes an output signal which regularly repeats high and low states by 8 times in one cycle which is divided into 16 parts in the example. For example, in a case where it is assumed that one pulse width is 10 μsec, one cycle is 160 μsec.
The light emitting element 62 is driven with pulses at least one time per one cycle by a light emitting element drive pulse (A). In a case of the existence of the detected object 63, the reflected light (B) from the detected object 63 is received by the light receiving element 52. As a result of input of an output pulse current signal from the light receiving element 52 into the IV conversion circuit 53, a positive voltage pulse signal is output from the light receiving amplifier circuit 54 (light receiving amplifier output (C)). Here, since the light receiving surface of the dummy light receiving element 57 is shaded, the voltage pulse signal is not generated in the output of the dummy amplifier circuit 59 (dummy amplifier output (D)).
The light receiving amplifier output (C) and the dummy amplifier output (D) are input to the differential amplification circuit 51 through the first high-pass filter circuit 55 and the second high-pass filter circuit 60, respectively, a difference voltage between the light receiving amplifier output (C) and the dummy amplifier output (D) is amplified by the differential amplification circuit 51 (differential amplification circuit output (E)). Here, in a case where in-phase noise, such as power supply line noise, is superimposed on an output line of the light receiving amplifier circuit 54 and output line of the dummy amplifier circuit 59, the in-phase noise is removed in phase by the differential amplification circuit 51. The output signal (differential amplification circuit output (E)) of the differential amplification circuit 51 is compared with the threshold voltage (F) by the comparator circuit 64. In a case where the output signal (differential amplification circuit output (E)) of the differential amplification circuit 51 exceeds the threshold voltage (F), a pulse signal is output from the comparator circuit 64 (comparator output (G)).
Furthermore, in the signal processing circuit 65, it is determined whether or not the comparator output (E) coincides with light emitting pulse timing (synchronous detection gate signal (H)). In a case where the comparator output (G) coincides with the light emitting pulse timing, the existence of the detected object is determined, and a result of the determination is output. In the signal processing circuit 65, the comparator output (G) and the synchronous detection gate signal (H), which is synchronous with a light emitting pulse, are respectively input to two input terminals of the AND circuit 65a. An output terminal of the AND circuit 65a is connected to an S input terminal SET of the SR latch circuit 65b, and the reset signal (L) is input to an R input terminal RESET.
A synchronous detection latch output (J) is inverted to a high level in a case where the comparator output (G) coincides with the light emitting pulse timing, that is, only in a case where both the comparator output (G) and the synchronous detection gate signal (H) become the high level in FIG. 10, and is initialized to a low level for each measurement cycle through input of the reset pulse signal (L). In this manner, the existence/non-existence of the detected object 63 is determined in each cycle. In the signal processing circuit 65, the existence/non-existence of the detected object is determined at any timing (for example, indicated by downward arrows in FIG. 10) based on the synchronous detection latch output (J). Specifically, in a case where the synchronous detection latch output (J) is at the high level, it is determined to be the “existence of the detected object” and a detection output (M) is set to the high level. In contrast, in a case where the synchronous detection latch output (J) is at the low level, it is determined to be “non-existence of the detected object” and the detection output (M) is set to the low level.
Meanwhile, contrary to the above case, in a case where the detection output (M) is set to the low level, it may be determined to be the “existence of the detected object” and in a case where the detection output (M) is set to the high level, it may be determined to be the “non-existence of the detected object”.
In addition, in the example, a case is illustrated where non-synchronous light sensing is performed in order to increase sensing accuracy. Even in a case where synchronous light is sensed and it is determined to be the existence of the detected object, it is assumed that non-synchronous light is detected in a case where a pulse signal is output as the comparator output (G) in a period in which the non-synchronous detection gate signal (I) is output. In this case, the result of determination that is the non-existence of the detected object (forced non-detection) as final determination non-determination (previous state is maintained) is output. In addition, similarly to the synchronous light detection, in the non-synchronous light sensing, the comparator output (G) is maintained as non-synchronous detection latch output (K) by the AND circuit 65c and the SR latch circuit 65d of the signal processing circuit 65. Therefore, in a case where a pulse signal of the comparator output (G) is sensed at least one time within the period in which the non-synchronous detection gate signal (I) is output, it is determined to be existence of the non-synchronous light.
In the example, a case where the output is determined per cycle is illustrated. However, in a case where the same determination is continuously acquired a plurality of times, in order to avoid a malfunction, the final existence/non-existence of the detected object 63 is sensed and determined and thus it is possible to improve sensing accuracy.
In addition, as the related art which is intended to improve sensing accuracy under the disturbance light, for example, PTL 2 discloses an object detection circuit which determines whether or not a pulse width of pulsed light sensed by the light receiving element coincides with (synchronous with) a pulse width of light radiated from the light emitting element using a pulse width sensing circuit.
Furthermore, PTL 3 discloses a technique which is capable of reducing erroneous sensing with respect to a case where minute reflected light and disturbance light from an object, which is not a sensing target, exist and there is a possibility of a malfunction occurring if a sum of the reflected light and the disturbance light is input to the light receiving element. PTL 3 discloses the following (1) to (3) as means for avoiding the malfunction due to the disturbance light.
(1) In a case where non-synchronous disturbance light is sensed, a malfunction due to the non-synchronous disturbance light is avoided by increasing a determination level of synchronous light.
(2) The malfunction due to the non-synchronous disturbance light is avoided by measuring a non-synchronous disturbance light level and increasing the determination level of the synchronous light in accordance with a measured value.
(3) In a case where a plurality of pulses are sensed in the synchronous light sensing, it is determined that a malfunction due to the disturbance light is generated.